In the past sixty years, the use of wireless and RF technology has increased dramatically, and in ways few could have foreseen, from limited military radar applications to today's ubiquitous penetration of wireless and microwave technology. The applications have expanded immensely but equally also have the volumes and customer base as applications such as RFID and cellular telephony have taken hold, but also in terms of functionality and complexity, and expectations of the consumers and users of these systems.
Today the plain old telephone for most people is now a portable, highly compact and light communications centre which provides not only telephony but also Internet access for email, web browsing and up-loading or downloading files together with music player, camera, and personal data assistant (PDA). But customers expect these with reduced cost, increased battery lifetime, and able to operate worldwide without intervention. As such, the cellular telephone is already required to operate on a number of different frequency bands to provide such worldwide usage of the cellular telephone. Moreover, with the drive for new cellular telephone features such as Global Positioning System (GPS) providing enhanced navigation and location fixing the ability to receive or transmit signal in other frequency bands will be required.
Naturally, the expanding capability of the cellular telephone must be achieved with an eye on cost, both from the user perspective but also the system operator or carrier. For the user, cost is normally a consideration of how much to pay for buying the phone and how many minutes per month at what fee. Power consumption of the cellular telephone is generally only factored by the consumer indirectly by how much talk-time or surf-time along is permitted between charging cycles. For the system operator, power is an important aspect of cost. The microwave infrastructure costs of the network include the cost of electrical consumption. Therefore, as more power is required, the more costly and the more difficult it is to provision not only in the context of a remote site, but also in today's urban environments where cellular density is increasing with microcells and picocells. In addition, power dissipation in the cellular telephone has important implications in the design of the cellular telephone and associated thermal management within the body or casing of the telephone.
An important aspect of this power dissipation is the efficiency of matching the microwave transmitter and receiver electronics to the cabling or antenna of the infrastructure and handheld wireless device. As a result, most electronic systems are designed to match one of a limited number of impedances such as 50 Ohms, generally used in microwave and RF applications, or 75 Ohm, as used within CATV, thereby removing for most the consideration of mismatches and wasted power. However, a free-space antenna's output impedance is typically 377 Ohm and is subject to variation based upon atmospheric effects, the proximity of the antenna to metal, and even the presence or absence of a user's head for cellular systems. A poor match often results in the transmission device purposefully increasing the output power of the amplifier to compensate for the lost power, thereby increasing overall power consumption.
Additionally, and generally not considered, is the variation in efficiency of the amplifier as its output power is adjusted even in a well-matched network. High efficiency in a power amplifier is achieved when the voltage swing across the output stage is at a maximum, often reaching the actual limits of the voltage supply. When this occurs very little power is dissipated by the output transistor devices, as when the transistor is conducting maximum current there is minimum voltage across it, and vice versa.
Now, when the transmitter output power is reduced, the output voltage swing across the output transistors is reduced. This results in a significant voltage across the output transistor devices when a significant current flows, and hence significant power dissipation.
In both examples, either the varying impedance mismatch or the varying output power, it is well appreciated by the community of designers involved in power amplifiers that there is an advantage in dynamically adjusting the impedance of a matching circuit within a microwave circuit. For the case of an impedance mismatch, such an impedance adjustment reduces the mismatch and hence wasted power. It should be appreciated that such an adjustment might be an increase or decrease in impedance. For the case of reducing the output power of an amplifier increasing the load impedance presented to the amplifier results in an increase of the output voltage swing, resulting in a smaller signal across the output transistor devices and therefore reduced power dissipation and increased efficiency.
It would therefore be advantageous in scenarios where power consumption is an important aspect of a network or handheld wireless device to remove the fixed nature of today's microwave circuits and systems and add a tunable impedance element. It would be a further advantage if the approach offered circuit designers a flexible design methodology to allow implementation within a wide range of circuits, was implementable in an extremely compact and low cost manner, and was compatible with integration to today's semiconductor microwave circuit technologies such as SiGe, GaAs, and InP as well as silicon.